Stammers vary hugely from person to person and situation to situation so I presume they will want you to fly with a CFI for an assessment of how you perform in practice. This assessment is common for physical issues but I haven't heard of it being required for speech issues before. Contact your local professional flying school and ask for a meeting with the CFI, he/she should know what to do and, if not, would contact the relevant national aviation authority (the CAA(if you're in the UK) for guidance. It won't be something you can or should prepare for and, if you've had any speech therapy, you will know that rehearsal is not a good thing.
I've had a life-long stammer and during my 30+ years in aviation have heard many others with a range of speech issues. I'm often slightly dis-fluent on the radio but the only time I really struggled was for a while when I was doing the MEIR course and I let the stress get to the vocal chords - normal service was restored after a while.
Ask yourself how well you perform doing presentations, interviews, debates or whatever - if you can communicate adequately you will be OK, but if you can't then it's better to find out earlier than later.